This invention relates to a central solar receiver, and, particularly, to a window for use in such a receiver for admitting and passing thereto concentrated solar radiation.
The present invention is particularly directed to a central solar receiver which comprises a housing with a solar absorber adapted to absorb concentrated solar radiation at high temperatures, commonlyxe2x80x94above 500xc2x0 C., and a working fluid circulating therein in indirect or direct heat exchange relationship with the solar absorber. The housing of such a solar receiver is formed with an aperture located in the focal region of a solar radiation concentration system, and holding a window adapted to admit highly concentrated solar radiation from the concentration system and to pass it towards the solar absorber.
The solar absorber used in central solar receivers of the above kind is often a volumetric solar absorber that is fabricated in the form of a three-dimensional matrix enabling the working fluid to flow therethrough, thereby transferring the heat to the working fluid. Such solar receivers are described, for example, in IL 97091 and U.S. Pat. No. 5,323,764.
The working fluid used in central solar absorbers normally either serves as heat carrier fluid or else is designed to perform a heat induced, possibly catalyzed, endothermic chemical reaction between components of the working fluid.
For various industrial applications such as the operation of gas turbines for electricity generation or the performance of endothermic reactions of the kind specified, it is necessary to enable the working fluid to circulate through the system at an elevated pressure of at least about 2 atmospheres. At such a pressure, the density of the circulating gaseous working fluid is higher than it would have been if it were in the non-pressurized state and, consequently, pressure losses during circulation are lower.
One of the most critical problems associated with a pressurized central solar receiver having a window, is the mechanical strength of the window. Materials that have required optical and thermal properties, tend to be brittle, which means that while they can withstand large compression stresses, they tend to crack or shatter under even relatively small tension stresses. Stresses in the window are created by the gas pressure in the interior of the receiver and also by uneven thermal expansion of the window and other receiver components that are in contact therewith, as they heat up during operation.
U.S. Pat. No. 5,323,764 and U.S. Pat. No. 5,421,322 disclose a central solar receiver with a window having a frusto-conical shape having a front large-diameter and rear small-diameter open ends both formed with cylindrical front and rear rims at which the window is secured to the housing at its front and rear ends. The front end of the housing is formed with an annular positioning groove and the front rim of the window is received therewithin and secured there by means of an O-ring. The rear rim of the window is secured within a metal block which inter alia includes thermal expansion absorbing bellows and a reflector adapted to protect the block and also the bellows from concentrated solar radiation entering the window.
It is an object of the present invention to provide a new window for use in a central solar receiver and a new central solar receiver using the same.
According to one aspect of the present invention, there is provided a central solar receiver comprising:
an axisymmetric housing having front and rear ends and having an aperture at the front end;
an elongated tubular window mounted in said aperture co-axially with the housing, the window having an open front end which is secured to the said housing at the front end thereof, and a closed rear end which is disposed adjacent the rear end of the housing and is free of any securing thereto, the window having a first surface facing incident concentrated solar radiation and a second surface facing the interior of the housing;
a volumetric solar absorber disposed within said housing and extending around and along said elongated window for absorbing solar radiation that has passed therethrough; and
working fluid ingress and egress formed in said housing so as to, respectively, inject thereto, and withdraw therefrom, a pressurized working fluid in a manner enabling the working fluid interaction with said volumetric absorber.
In accordance with another aspect of the present invention, there is provided a window for use in a central solar receiver of the above kind, for admitting and passing into the receiver incident highly concentrated solar radiation, the window having an elongated tubular shape with an open front end adapted for being secured in said aperture of the receiver, and a closed rear end free of any securing means.
The design of the window of the present invention and the manner of its mounting in the central solar receiver, whereby the window is secured only at its front end, simplify the receiver""s design and prevent the window from having undue constraints, which renders it suitable for use at high temperatures and elevated pressures.
The window is preferably of a frusto-conical shape, with its open front end being a large diameter end and its closed rear end being a small diameter end. However, the window may have any other elongated axisymmetric shape, e.g. a cylindrical shape, in which case its front end may be formed with a rim having a diameter greater than that of the remainder of the window.
Preferably, the window""s front end is secured to the housing by elastic mounting devices such as, e.g. spring-loaded clamps, that permanently exert on the window""s body axial force that keeps the window in place and prevents its motion under forces acting thereon during thermal and pressure cycles when the receiver is operated.
Preferably, the rear closed small-diameter end of the window is in the form of a cap-like concave surface continuously merging with the window""s conical surface. This shape of the closed end, and the frusto-conical shape of the window""s body, ensure that any pressure exerted on the window from within the housing produces force components acting perpendicular and along the window""s surface, which only results in compression stresses rather than tension stresses that may cause the window to crack.
Preferably, the working fluid ingress is located adjacent the rear end of the housing co-axially therewith so as to face said closed end of the window. It is still more preferable that an additional solar absorber is mounted in the housing between the working fluid ingress and the closed end of the window so as to absorb concentrated solar radiation that has passed through the closed end. Thereby, the working fluid ingress is protected from direct penetration thereto of concentrated solar radiation and is pre-heated, while entering the receiver, before its interaction with said volumetric solar absorber. The additional solar absorber may also be a volumetric solar absorber.